In my area:

1. My area is a relatively Low cost of living area surrounded by people that have money. When they want to work remote but still stay in the same state, some % come east for cheap housing. They can pay cash for houses.
2. Corporations are buying houses, then renting. Less houses on the market. They also pay cash for houses.
3. People with multiple houses can use the aforementioned houses to buy up more housing to rent, creating a investment loop. Its in their best interest to keep the houses as small businesses/revenue streams.
4. There are apps that hook into MLS (example: www.mlslistings.com/more/mobile-app/) which means investors can make an offer faster than the general public or set up alerts (think stock market) that hook into the API. Ever wonder why some houses get multiple offers before or right after they are listed? This only works if the company/entity selling the property decides to add in the info into MLS and not just go with the defaults.
5. More people are alive today than yesterday. They need housing. There are less houses getting built, or being built for owning. For renting: www.youtube.com/watch?v=mrxZqPVFTag

Source: Used to work for a company that did this sort of buy/selling of properties.